Summary:
SharedShadowNodeList is one of the core collections in Fabric. Every ShadowNode has one, it's used intensivly during diffing and so on.
Usually, nodes have a very few children, so using `small_vector` instead of regular `vector` should save us a lot of memory allocations.
Reviewed By: JoshuaGross
Differential Revision: D14249201
fbshipit-source-id: 53297caa027a74099d0a1ed4a3cce78f8feb651b